1. Field of the Invention
The present invention relates to an image sensor and a method of driving the same and, more specifically, to an image sensor and a method of driving a transfer transistor of the image sensor to reset a photodiode and transfer charge in the photodiode.
The present invention has been produced from the work supported by the IT R&D program of MIC (Ministry of Information and Communication)/IITA (Institute for Information Technology Advancement) [2005-S-017-02, Integrated Development of UltraLow Power RF/HW/SW SoC] in Korea.
2. Discussion of Related Art
Image sensors may be largely divided into charge-coupled device (CCD) image sensors and complementary metal oxide semiconductor (CMOS) image sensors. Both the CCD image sensor and the CMOS image sensor use electron-hole pairs generated by light having a greater energy than the bandgap energy of silicon. Conventionally, the CCD and CMOS image sensors utilize a technique of estimating the quantity of irradiated light by collecting electrons or holes.
Like other CMOS devices, a CMOS image sensor includes a photodiode and a transistor disposed in each photosensitive pixel so that the CMOS image sensor can be fabricated using a conventional CMOS semiconductor fabrication process. Thus, as compared with a CCD image sensor in which an image signal processor should be disposed in an additional chip, the CMOS image sensor can integrate an image signal processor and an image detector into a circuit block disposed outside a pixel, operate at low voltage, and be fabricated at low production cost.
Conventionally, CMOS image sensors may be classified into a 4-transistor pixel structure and a 3-transistor pixel structure according to the number of transistors that form a single photosensitive pixel. Although the 3-transistor pixel structure is advantageous in terms of the fill factor and production cost, the 4-transistor pixel structure is widely used because the 4-transistor pixel structure separates a light receiving device from a detection unit, and the light receiving device is formed of a silicon bulk except for the surface thereof so that the 4-transistor pixel structure is highly responsive to light and resistant to dark current and noise.
A conventional 4-transistor pixel structure is illustrated in FIG. 1. The 4-transistor pixel structure includes four transistors. Specifically, a photodiode (PD) functioning as a light sensor and four NMOS transistors constitute a unit photosensitive pixel. The four NMOS transistors include a transfer transistor Tx, a reset transistor Rx, a drive transistor Dx, and a switch transistor Sx. The transfer transistor Tx functions to transfer photocharge generated by the photodiode PD to a diffusion node region FD, the reset transistor Rx functions to emit charge stored in the diffusion node region FD or the photodiode PD to detect signals, the drive transistor Dx functions as a source follower transistor, and the switch transistor Sx functions to perform switching/addressing operations.
The photodiode PD and a capacitor 118, which are connected in parallel, constitute a light receiving device, and the transfer transistor Tx transfers electrons generated due to photons to a diffusion node 131. In order to obtain a 2-dimensional image, an electric potential is applied through a gate 141 of the switch transistor Sx to select a column. In particular, each photosensitive pixel is biased by a current source 150. The current source 150 drives the drive transistor Dx and the switch transistor Sx to read the electric potential of the diffusion node 131 from an output node 142.
FIG. 2A is a cross-sectional view of a photodiode and a transfer transistor of a conventional 4-transistor CMOS image sensor.
Referring to FIG. 2A, an n-doping region 202 having a predetermined concentration and a p+ region 203 functioning as a surface pinning region are disposed on a p-type substrate 201 and constitute a photodiode, which is a light receiving device. A gate insulating layer 205, a gate electrode material 206, a control line 210, and a sidewall insulating layer 207 are disposed on the surface 201 and constitute a transfer transistor. The transfer transistor is used to reset the n-doping region 202 in which photocharge is generated and accumulated and to transfer the photocharge. In this case, diffusion nodes 204(a) and 204(b), which serve to convert photocharge into a voltage, include a diffusion region 204(a) that is doped with n-type impurities before forming the sidewall insulating layer 207, so that the diffusion nodes 204(a) and 204(b) can be self-aligned with the gate electrode material 206 of the transfer transistor.
FIG. 2B is a signal waveform diagram illustrating methods of driving a transfer transistor and a reset transistor to transfer photocharge generated by a photodiode and reset the photodiode in a conventional 4-transistor image sensor. Typically, a power supply voltage Vdd is used as a turn-on voltage of each of a transfer transistor and a reset transistor, and a ground voltage is used as a turn-off voltage thereof. When a reset transistor Rx is turned on (refer to 231), low impedance is maintained between the photodiode and a drain of the reset transistor Rx during a turn-on period 232 of a transfer transistor Tx, so that charge accumulated in the photodiode is emitted out of a photosensitive pixel to reset the photodiode. After resetting the photodiode, a diffusion node is reset during a turn-on period 235 of the reset transistor Rx and thus, a voltage of the diffusion node is pinned to a voltage obtained by subtracting a threshold voltage Vth of the reset transistor Rx from the power supply voltage Vdd. After the reset period 232 of the photodiode, the photodiode receives light. Thus, photocharge, which is accumulated in the photodiode during generation/accumulation period of photocharge, i.e., an integration time 236, is transferred to a diffusion node constituting a source follower and finally converted into a voltage at an external circuit when the transfer transistor Tx is turned on (refer to 233). In this case, the intensity of light is detected due to a drop in the voltage of the diffusion node read from an output node after the period 233 for transferring the photocharge on the basis of the voltage of the diffusion node read from the output node after the period 235 for resetting the diffusion node.
Therefore, in operation of the 4-transistor pixel CMOS image sensor, photo-generated carriers accumulated in the photodiode after the reset period 232 of the photodiode are transferred to a floating diffusion node so that the amount of the photo-generated carriers is detected due to a drop in the voltage of the diffusion node. Therefore, it is necessary for the transfer transistor to perform constant and uniform reset and transfer operations in order to precisely and uniformly detect the amount of the accumulated photo-generated carriers. A conventional 4-transistor pixel CMOS image sensor includes various photodiodes, such as a complete-reset pinned photodiode, so that the transfer transistor performs constant reset and transfer operations. When the complete-reset pinned photodiode is reset, all mobile charge in the photo diode is completely depleted so that no variation in electric potential occurs. In this case, the electric potential of the photodiode may be always pinned to a constant value irrespective of external bias environment such as the electric potential of the floating diffusion node. Thus, the transfer transistor can always perform the reset and transfer operations under uniform and equal conditions.
However, in recent years, the electric potential of a diffusion node has been increasingly lowered in order to downscale semiconductor devices and reduce power consumption. Due to a reduction in the electric potential of the diffusion node, the pinning electric potential of a complete-reset pinned photodiode is naturally dropped. In this case, however, pixel characteristics, such as well capacity and the responsivity of a photodiode to light, may worsen and fixed pattern noise may increase. As a result, even if an operating voltage is reduced, there is a limit to dropping the pinning electric potential of the pinned photodiode.
In a typical pixel driving condition where the power supply voltage Vdd is equal to a turn-on voltage, the conditions under which the photodiode performs reset and transfer operations are changed. In order to reset the photodiode, when a reset transistor is turned on, the voltage of the diffusion node is pinned to a difference Vdd-Vth between the power supply voltage Vdd and the threshold voltage Vth of the reset transistor. Since a channel of the reset transistor has about the same dopant concentration as a channel of the transfer transistor, when the transfer transistor is turned on, a voltage difference between the gate electrode of the transfer transistor and the diffusion node becomes the threshold voltage Vth so that the diffusion node may operate under boundary conditions between pinch-off conditions and linear conditions. Thus, the gate voltage of the transfer transistor may be applied and electrons may be instantaneously emitted from the diffusion node to the channel of the transfer transistor, thereby greatly affecting the reset and transfer conditions of the photodiode. Also, the influence of the diffusion node is very sensitive to process variables (Bongki Mheen, et. al., “Operation Principles of 0.18-μm Four-Transistor CMOS Image Pixels With a Nonfully Depleted Pinned Photodiode,” IEEE Trans. Electron Devices, vol. 53, no. 11, 2006).
During the reset and transfer operations of the photodiode, the moment a turn-on voltage is applied to the transfer transistor, a region disposed under a gate of the transfer transistor becomes a deep depletion region irrespective of the physical magnitude of the gate of the transfer transistor or the operating voltage. The instantaneous deep depletion region formed under the transfer transistor induces more charge to be emitted from the diffusion node toward the channel of the transfer transistor than when the channel of the transfer channel is in a state of stable equilibrium. In other words, even more charge may be emitted from the diffusion node than when the channel of the transfer transistor is in the state of stable equilibrium, and the amount of charge emitted from the diffusion node may be affected by a method for applying a voltage or the physical structure of the transfer transistor.
Also, the influence of the diffusion node may depend on whether the photodiode undergoes a reset operation or a transfer operation. After resetting the photodiode, the diffusion node is floated. Unlike when the photodiode is reset, the voltage of the floating diffusion node is pinned to a voltage obtained by subtracting the threshold voltage of the reset transistor and a voltage due to a clock feedthrough effect caused by the turn-off of the reset transistor from the power supply voltage. Also, the voltage of the floating diffusion node becomes closer to linear conditions than when the photodiode is reset. However, as the voltage of the transfer transistor rises due to a coupling capacitance present between the transfer transistor and the floating diffusion node, the voltage of the floating diffusion node also rises. Furthermore, since the instantaneous emission of electrons to the channel of the transfer transistor affects the voltage of the floating diffusion node again, the extent of the influence of the diffusion node depends on whether the photodiode performs the reset operation or the transfer operation (Bongki Mheen, et. al., “Operation Principles of 0.18-μm Four-Transistor CMOS Image Pixels With a Nonfully Depleted Pinned Photodiode,” IEEE Trans. Electron Devices, vol. 53, no. 11, 2006).
In a conventional case where a higher voltage than a pinning voltage is used as an operating voltage, a photodiode is more fully reset so that reset and transfer operations of the photodiode may be hardly affected by a diffusion node. As a result, the influence of the diffusion node on reset and transfer conditions of the photodiode can be excluded. However, although the operating voltage is sharply reduced due to the downscaling of the semiconductor devices and low-voltage operation conditions, the threshold voltage of the transistor cannot be dropped below the limit. Thus, during the reset and transfer operations of the photo diode, the pinning electric potential of the photodiode should drop more sharply in order to prevent charge from flowing from the photodiode to the channel in a subthreshold region (i.e., in order to completely deplete the photodiode in a short amount of time). Even if the pinning electric potential of the photodiode is lowered by sacrificing light responsivity or well capacity, since the influence of the diffusion node on the photodiode depends on whether the photodiode performs the reset operation or the transfer operation, the pinning electric potential of the photodiode should be reduced still more in order to pin the photodiode at a constant voltage level. Furthermore, because the influences of process variables and drive methods need to be considered, it becomes more difficult to determine the pinning voltage or physical structure of the photodiode.
Also, a predetermined potential barrier is present between the transfer transistor and the pinned photodiode on which a p-type doping layer serving as a surface pinning layer is formed. In order to eliminate the influence of the potential barrier on the reset or transfer operation of the photodiode, the pinning electric potential of the photodiode, the electric potential of the floating diffusion node, and the turn-on electric potential of the transfer transistor should be sufficiently different. When the potential barrier is not sufficiently reduced, even if the pinning voltage of the photodiode is very low, the photodiode is not completely reset and the amount of charge remaining in the photodiode during the reset and transfer operations is determined by the potential barrier, thereby causing serious problems. Specifically, as the operating voltage is reduced, a difference between the pinning electric potential of the photodiode and the electric potential of the floating diffusion node is also reduced, it is strongly likely that well capacity will be generally lowered and the photodiode will be incompletely reset, and the influence of the diffusion node becomes very sensitive to process variables.
In order to overcome the above-described drawbacks, several conventional methods have been proposed. First, a voltage applied from a floating diffusion node to a gate of a reset transistor Rx may be forcibly boosted from a general electric potential VDD-VTH to a power supply voltage VDD using a voltage boosting circuit. Second, a PMOS transistor may be used as a reset transistor Rx instead of a conventional NMOS transistor so that the electric potential of a floating diffusion node can be boosted to a power supply voltage VDD.
However, when using the voltage boosting circuit, a higher voltage than a typical operating voltage is applied to the gate of the reset transistor Rx so that the reliability of a gate oxide may deteriorate. Also, when the PMOS transistor is used as the reset transistor Rx, the PMOS transistor occupies a larger area than the NMOS transistor so that the fill factor is reduced to degrade the characteristics of the reset transistor Rx. Furthermore, it is known that the PMOS transistor doubles the noise of the NMOS transistor.
In addition, the above-described approaches do not fundamentally solve the problems caused by a low operating voltage, but they are just aimed at elevating efficiency at the same operating voltage.
Therefore, various methods have conventionally been disclosed to solve the foregoing problems. For example, in Korean Patent Registration No. 10-059175 entitled “Active Pixel Sensor Using Transmission Transistor Having Coupled Gate”, two transfer transistors may be included so that after one transfer transistor adjacent to a photodiode is turned off and floated, another transfer transistor is turned on to boost a voltage using coupling capacity. In another method, a voltage applied to a gate of a transfer transistor may be boosted using a coupling capacitor obtained by forming a conductive layer on a transfer transistor Tx by interposing an insulating layer therebetween. Furthermore, Korean Patent Laid-open Publication No. 10-2006-0084484 has introduced a method using an effect obtained by boosting a voltage of a diffusion node FD to as much as a coupling voltage by forming a coupling capacitor including a conductive layer formed on a drive transistor Dx by interposing an insulating layer therebetween.
However, the method of boosting a voltage using coupling capacity in the transfer transistor involves turning one transfer transistor off to float the photodiode and the transfer transistor. Thus, a voltage is applied again to a coupling capacitor so that the voltage is transmitted to a channel of the transfer transistor by as much as a ratio of coupling capacity to the entire capacity. Therefore, photocharge, which is emitted from the photodiode to the channel of the transfer transistor before boosting a gate voltage due to the coupling capacity, flow into a substrate or the photodiode during the boosting of the gate voltage, thereby degrading the transmission efficiency of the photocharge. Also, a voltage boosting effect is changed due to a time taken until the voltage is applied again to the coupling capacitor disposed on the transfer transistor after removing the voltage applied to the gate of the transfer transistor. Accordingly, the boosting effect may greatly depend on a method of applying a voltage or an immaterial time error. Moreover, the photodiode is reset under other conditions than when photocharge accumulated in the photodiode is transferred, thereby precluding generating constant dark current or sensing light having specific luminous intensity or less.
Above all, in the above-described method, a voltage applied to the transfer transistor is boosted in order to elevate the electric potential of the photodiode to a constant high electric potential in a short amount of time during the reset or transfer operation of the transfer transistor. However, while the voltage applied to the transfer transistor is being boosted, more electrons are emitted from the diffusion node to the channel of the transfer transistor during the reset or transfer operation of the photodiode. As a result, the emission of charge accumulated in the photodiode may be controlled. Also, the influence of the diffusion node on the emission of the charge accumulated in the photodiode may depend on whether the transfer transistor performs the reset operation or the transfer operation. Therefore, as a higher voltage is applied to the transfer transistor, the entire noise of an image sensor may increase.